1. Field of the Invention
The present invention relates to a hybrid vehicle, and more particularly to a parallel hybrid vehicle.
2. Description of the Related Art
Generally known parallel hybrid vehicles have an internal combustion engine as a primary propulsive power unit and an electric motor, more accurately a generator motor which can also operate as an electric generator, for generating an assistive output power for assisting the output power produced by the engine. The output powers (mechanical powers) generated by the engine and the generator motor are transmitted through a transmission to drive wheels of the hybrid vehicle.
For accelerating the hybrid vehicle, the generator motor is controlled to generate an assistive output power, and both the output power produced by the engine and the assistive output power from the generator motor are transmitted to the drive wheels. Therefore, the power requirements for accelerating the hybrid vehicle can be met, and the output power produced by the engine may be relatively small, thus reducing fuel consumption by the engine and exhaust gases emitted from the engine.
When the engine is idling while the hybrid vehicle is not running or when the output power produced by the engine does not need to be assisted while the hybrid vehicle is running, e.g., while the hybrid vehicle is cruising, the output power produced by the engine is wholly or partly used to cause the generator motor to operate in a regenerative mode, and the electric energy generated by the generator motor is stored in an electric energy storage unit such as a battery used as a power supply for the electric energy.
On the hybrid vehicle, it is necessary to supply electric energy to various accessories including an ignition system for the engine, an air-conditioning system and an audio system on the hybrid vehicle, a controller for controlling the generator motor, and other electric components. The voltage at which to energize these accessories is lower than the voltage at which the generator motor is operated. Therefore, the voltage of the electric energy generated by the generator motor and the voltage of the electric energy stored in the electric energy storage unit are lowered from 100-180 V to about 12 V by a DC to DC converter, and the electric energy of the lowered voltage is supplied to the accessories and also stored in a battery for the accessories for subsequent usage by the accessories.
The DC to DC converter, however, lowers the efficiency with which to generate the 12 V electric energy based on the electric energy generated by the generator motor, to 40%-60%. One solution would be to use a 12 V electric generator for generating and supplying 12 V electric energy to the accessories and also storing the 12 V electric energy in the battery for the accessories, so that the efficiency with which to generate the 12 V electric energy will be increased by not lowering the voltage of the electric energy generated by the generator motor to 12 V. However, the 12 V electric generator used in addition to the generator motor would require some means, e.g., a pulley and belt mechanism or a system of gears, for transmitting kinetic energy from the engine, the generator motor, and the drive wheels to the 12 V electric generator. As a result, the overall arrangement of the hybrid vehicle would be complicated in structure and increased in size.
On some hybrid vehicles, the output shaft of the engine and the rotatable shaft of the generator motor coaxially coupled to each other at all times. The generator motor regenerates electric energy with the output power from the engine based on the rotation transmitted directly from the output shaft of the engine, and assistive output power from the generator motor is transmitted directly to the output shaft of the engine.
On these hybrid vehicles, the generator motor regenerates electric energy highly efficiently when the rotational speed of the rotatable shaft of the generator motor ranges from 2000 to 3000 rpm. However, while the hybrid vehicle is being normally driven, the most frequently used range of rotational speeds of the engine is from 600 to 1800 rpm, and while the engine is idling, its rotational speed is 400 rpm in a vibration-reducing control mode in which the generator motor regenerates electric energy and produces output power alternately depending on torque fluctuations of the engine. Therefore, the usual range of rotational speeds of the engine is lower than the range of rotational speeds of the generator motor for highly efficient electric regeneration. Because the output shaft of the engine and the rotatable shaft of the generator motor coaxially coupled to each other at all times, the generator motor not only fails to regenerate electric energy highly efficiently, but also dissipates a large amount of heat due to the electric regeneration and the engine output power assistance that are affected in the less efficient rotational speed range. Furthermore, the fuel consumption by the engine is increased when the vibration-reducing control mode is carried out in the less efficient rotational speed range.
The assistive output power generated by the generator motor is transmitted, without being amplified, to the output shaft of the engine. Consequently, if a relatively large assistive output power is required for starting or accelerating the hybrid vehicle, the generator motor has to be relatively large in size and hence in weight.